I think your players improved. The message means you didn't gain anything towards a bonus (the ones you can use in match).
yeah, probably that happened.
In another training, I set the "warm up" drill first and then other five hard or very hard drills and the team bonus raised normally.
So, we can say that if we want to power train one ore more players, we must start with an easy drill (like warm-up or stretch) and then set 5 hard or very hard drills or else maybe we 're gonna loose the team bonus from using wrong sequence of drills.